Method fob making bituminous paving mixtures



, F. s. BESSON METHOD FOR MAKING BITUMINOUS PAVING MIXTURES Filed May 26 1921 @nk SQ/Tg new Patented Aug. 7, 1923.

EFRANK S. BESSON, F WASHINGTON, DISTRICT OF COLUMBIA.

METHOD ron :MAKING BITUMINOUS PAVING M'Ix'ruans.

Application filed May 26, 1921. Serial No. 472,889.

(FILED UNDER THE ACT 0F MARCH 3, 1883, 22 STAT. 12M-625.)

To all whom t may cof/wem:

Be it known that I, FRANK S. BESSON, a citizen of the United States, and a resident of Washington, District of Columbia, have invented an Improvement in Methods for Making Bituminous Paving 'Mixtures, of

which the following is a specification.

The invention described herein may be lused by the Government, or any of its oflicers or employees in prosecution of work for the Government, or by any other person inthe United States, without payment to me of any royalty thereon.

rllhis invention relates to paving mixtures,

more particularly to'a new and improved method by which the bituminous binding agent and mineral aggregate are correctly proportioned and combined to produce an eflicient mixture for highways, sidewalks,

2@ floors and similar constructions.

Une of the principal purposes of this invention is to provide a formula for paving mixtures, which will aiord uniform paving results and permit an accurate determi- 25 nation of the bituminous requirements for any character of mineral aggregate.

Another important object of this invention is to provide a formula which will enable the bitumen requirement for any given quantity of mineral aggregate to be proportioned in accordance with the void percentage and specific gravity of the'aggregate.

Another important ob]ect of this invention is to produce a paving mixture characterized by its wear resisting qualities and ability to resist deterioration, crawling, creeping, undue deformation or fracture when subjected to traffic wear or adverse moisture and heat conditions.

With these and other objects in view this invention consists in certain novel combining and proportioning of materials as will be hereinafter more fully set forth and claimed.

In attempting to solve the' problem or producing a successful paving mixture, high' way engineers have ladvanced and resorted to many theories and formulas, some advocating surface area of aggregate, the mechanical analysis, accumulative` plat, or ineness modulus methods; while others have attached considerable importance to density measurements of samples taken from streets, or have contended that wavy pavements originate in the use of sand or mineral aggregate the grains of which are round and smooth instead of angular and rough.

Actual experiments,'however, prove the fallacy of these theories, asmixtures made in accordance with the same formulas vary in many respects, one resultant surface being excellent, another corrugated and waved, and still another cracked and disintegrated.

In an endeavor to find a solution to this problem many experiments have been made and the result of these tests ndicatethat defective pavements are often traceable to an improper proportion of bitumen in the mixture.v It is therefore the aim and purpose of this invention to provide a formula which will enable the correct amountof bitumen for any particular aggregate to be accurately determined, andy in this connection it is to be understood, that the term aggregate as used hereinafter in the description and claims, embraces the total mineral matter of the mixture.

In the accompanying drawing, the figure is a graphical representation ofthe relationship between bitumen requirements, aggregate voidage, specific gravity, and resulting satisfactory paving.

Coming now to the subject matter of the present invention, numerous experimental tests with samples of existing pavements gave definitely established the following acts:

(a) That sieve analyses of mineral aggregates do not afford an accurate basis for determining bitumen requirements.

(b) That specimens of imperfect surfaces prepared by sieve analysis formulae supposedly correct, indicate, according to the defective conditions of the paving, either too much or insufficient bitumen, as required by the void percentage and specilic gravity of the aggregate.

(c) That definite relations exist between the bitumen requirements, void percentage, and specific gravity of the aggregate.

(d) The proportion by weight of bitumen to aggregate depends upon the volume of a given Weight of aggregate.

1With the preceding facts established many investigations were made of specimens having a denite volume (one thousand cubic centimeters) taken from Washington thoroughfares, which had rendered satisfactory service over a number of years; each specimen being analyzed to determine- (a) Bitumen contents (b) Void percentage and specific gravity of aggregate.

In making these determinations the volume measurements may be made by means of a graduated cylinder 62 millimeters in diameter and of suiiicient capacity to h'old a thousand grams of material.

After obtaining the bitumen contents of the specimens and the void percentages and specific gravities of the aggregates, a chart is plotted wherein the curve (2) represents the resultant of the abscissas (l) andthe ordinates (3) and (4) the specific gravitics and bitumen contents expressed in percentages by weight.

With this diagram at hand, to determine the bitumen requirements or' an original mixture, the operator determines the void percentage and specific gravity of a thousand grams of dry loose aggregate by any of the usual methods for ascertaining voidage and density of an aggregate, and then with these factors known, consults the chart tor the correct amount of bitumen shown by the previously analyzed specimens to be required for an aggregate having a similar void percentage and specific gravity.

To obtain the speciiic gravity ordinates (3) it is noted that the sloping axes of these ordinates indicate that the bituminous percentages increase inversely as the specific gravity of the aggregates. Thus the verti- -cal axis of ordinates representing percentages for aggregate of specific gravity 3, Aforms with the 2.3 axis an angle Whose cosine is 2.3 divided by 3, and this angle is ap roximately i0 degrees eference being vhad to the accompanying dra-win it will be seen that, having ascertained or any particular aggregate the void percentage indicated by ordinates 1, a point may be located on the curve 2, and corresponding to this point on the specific gravity ordinates 3, may be .read from ordinates 4 the. percentage by weight of bituminous cement testine1 approximately 99%v bitumen soluble in car" on bisulphide, the bitumen be'- ing oi approximately 1.06 specic gravity.

lin order to facilitate estimates, ordinates 5 may he added to and used in coniunction with the diagram for determining the amount of satisfactory pavement resulting per cubic yard oi? aggregate used.

As a more specific illustration of the application of the accompanying diagram, assuming the percentage of voids to be 41.0 for 'a given aggregate, having a specific gravity of 2.? g upon entering the diagram with the void percentage ot the particuiar aggregateas an abscissa, a point a is located on curve 2 which corresponds to the point on the specidc gravity ordinates, thus giving a incense reading of 8.2 as the correct percentage of bitumen required for the mixture in the event ordinates 5 are included in the diagram, a point c may be located in this ordi-r nate column which corresponds to the point a on curve 2 and gives a reading of approximately 29.2 square yards of pavement l thick per cubic yard of aggregate.

ln the illustrated embodiment characterizing this invention reference has` been made to bituminous cement testing 99% soluble in carbon b isulphide, the bitumen being of approximately 1.06 specilic gravity; it is to be understood, however, that the relationship between the void percentage, specific gravity oi the mineral matter and amount of bitumen soluble in carbon bisulphide exists irrespective of the character of bituminous cement used, and that proper alteration may readily be made to render the diagram applicable to otherbituminous cements, these alterations being made according to the variance in the degree of solubility and speciiic gravity of the bitumen.

Although in the preceding description a .definite method has vbeen outlined i'or determining the correct amount or' bitumen tor any particular aggregate dependant on the void percentage and specilic gravity ot the latter, it is to be understood that other methods may be employed for accomplishing this same result without departing7 from or sacrificing any of the principles of this invention.

In concluson, it may be stated that, as this invention by properly proportioning the components of the paving mixture, eliminates Waves, cracking, yand other unsatisfactory paving results, the binder course ordinarily utilized for overcoming these detects is made unnecessary; in consequence of which a material @ving is eeeted in the cost of concrete constructions et the character under discussion.

. Havin described my invention, whatll lclaim to le new and wish to Aprotect by Letters inatent is:

A method of making bituminous paving mixtures, consisting in determining the void percentage of a definite volume'ot a loose aggregate and the specific gravity ott the aggregate, with these as rfactors determining from the graph illustrated in the accompanying drawing the amount 'of bitumen required for an aggregate having similar void percentage and specific gravity and adding this amountv of bitumen to the loose aggregater, such graph being based upon and representing the results Lor the determinations of the bitumen content, void percentage and specific gravity of a series of selected, diering specimens of satisfactory paving mintures.

S., BESSQN. 

